Warning: this is a subtle effect, and you have to calibrate your browser.

In order to appreciate this, please proceed in the following order:

Test your sound output by pressing the button ‘Soundcheck’ (or hit key ‘s’). You should hear something of a ‘tchuck’ sound. If not, up your sound volume or go to another illusion :).

Calibrate the sound delay of your browser* as follows: press the button labeled ‘1’ (or hit key ‘1’). The blue balls should begin to move smoothly. At the very moment they meet, the ‘tchuck’ sound should occur. Use the long vertical slider on the right to adjust the timing, if the sound occurs too late (or too early).

Now you can try out the actual phenomenon like so:
Compare the effects after pressing buttons ‘1’ or ‘2’ ad lib by determining the paths of the blue balls. Are they crossing like so ‘X’, or do they bounce off each other ‘> <’?

Chances are: When there is no accompanying sound, the crossed path is seen, with the sound a bounce (‘> <’) is more likely to be perceived.

Comment

This is a crossmodal illusion, where sound affects vision. It was long known from the Gestaltists that two identical visual targets moving across each other can be perceived either to bounce off or to stream through each other (Metzger 1934). In 1997 Sekuler et al. demonstrated that a brief sound at the moment the targets coincide, biases perception toward bouncing.